1. Field of the invention
The present invention relates to a variable resistance circuit, and, more particularly, to a variable resistance circuit the resistance value of which is varied electronically.
2. Description of the prior art
Various variable resistance devices have been known heretofore to obtain a adjustable resistance value across two terminals thereof. One known device is a variable resistor whose resistance value is mechanically varied in response to the manipulation of the slider contact. Such variable resistor, however, has disadvantages in stablity and reliability in its operation due to the abrasion of and the adhesion of dust to the slider contact. Since the resistance value is mechanically varied in itself, it is difficult to electronically vary the resistance value. Further, such variable resistor inserted in a signal path degrades a signal-to-noise ratio thereof. Another known devices are variable resistance circuits such as a voltage controlled amplifier (hereinafter called VCA) and a circuit using variable characteristics of conductiveness of semiconductor devices such as field effect transistor (hereinafter called FET). However, the VCA has the restriction in its use, that is, the VCA cannot be used as a variable resistance device in a floating condition across power supplies. On the other hand, in the circuit using FET, the allowable input voltage is low due to the characteristics of FET device in itself, and the circuit has a drawback to develop a distortion due to the non-linearity characteristics of FET. In addition to this, the circuit is normally the one that the resistance value against the ground is varied, and accordingly has such restriction in its use as the VCA.